The invention relates to a control circuit for resistance wire heaters used to de-fog or de-ice windows of automobiles, or other structures.
It is conventional in present day automobiles to provide an electrical resistance grid in or on the rear window of the automobile for heating the glass sufficiently to vaporize fog thereon or to melt ice or snow that may accumulate thereon. Thus fog (extremely small droplets of water that obscure vision through the glass) or ice may be removed sufficiently to restore transparency.
Such electrical resistance heaters are actuated by a timer switch which may be turned on manually when the driver notes that the window has become obscured, and which then automatically turns off after a pre-determined time interval, found by trial and error to be sufficient to allow the heater to vaporize fog or melt that amount of ice that normally may be expected to collect on the window. But such conventional controls, that energize the heater only for a fixed, arbitary time interval, usually "overdo" the job in that the time during which they are turned "on", and are using current from the car battery/alternator, must be greater, than the actual time necessary to vaporize the fog or melt that amount of snow/ice that could be expected to collect on the window under normal conditions. If it were otherwise, instances frequently would occur where the time control would turn the heater off before the de-fogging or de-icing was completed.
Therefore, attempts have been made in the past (see for example U.S. Pat. Nos. 2,006,006; 2,470,633 and 2,507,036) to provide controls for window de-fog or de-ice heaters which are not controlled simply by timers, but by other means. However, none of these attempts have been successful in meeting the needs of the automotive market because the straightforward time control still is used commercially, with resultant waste of current (energy) and with the imposition of unnecessary loads on the battery/alternator system of the vehicle.